While magnetooptic recording media capable of storing information with high densities are expected to come into wide use as large-capacity external memories, of these media an optical disc is attracting notice as an external memory for computers on the ground that it can effect the playback at a high speed. While there have been proposed various kinds of magnetooptic recording media which are different information storing method and size, in accordance with the ISO standards, the magnetooptic recording medium of the write-once type capable of writing information only once and the magnetooptic type capable of erasing the information have been standardized with respect to the size of 5.25 inches in diameter, whereas the ROM type and the magnetooptic type used exclusively for playback purposes as well as the partial ROM type featuring the coexistence of the magnetooptic type and the ROM type have been standardized with respect to the size of 3.5 inches in diameter, and these types are expected to be put on a still wider market in the future.
Also, recently the application of the optical disc has also been started in the field of digital audio. For instance, players have been put on the market in which the write-once type optical disc or magnetooptic disc is used as a master source in a digital multiple track recording of 24 to 48 tracks. In addition to these products adapted for use by the specialists, there has been the advant of products adapted for use by the general consumers, such as, a CD-R (a compact disc of the write-once type) and an MD (miniature disc) and their future trend attracts notice.
In these optical discs, the guides for orderly arranging the information marks by the laser beam from an optical pick-up of a recording and playback apparatus, i.e., the guides for tracking purposes are formed in the form of recessed or raised grooves spirally from the inner periphery toward the outer periphery of the disc. These grooves are called as guide grooves.
Explaining the guide grooves in greater detail, as defined according to the ISO standards, those portions constituting the recesser or the far portions as looked from the optical pick-up are called as lands and the other portions constituting the raised portions or the near portions as looked from the pick-up are called as grooves. Information is recorded on either the lands or the grooves. When recorded on the lands, it is called as a land recording system, whereas when recorded on the grooves, it is called as a groove recording system. The paths for recording the information are called as tracks, and the spacing between the center of one track and the center of the adjacent track is called as a track pitch.
Generally, each of the tracks is further divided into a plurality of portions which are called as sectors and each of the sectors is composed of an address part, a flag part, a data part and a buffer part. The address part indicates a physical address on the disc, the flag part indicates whether the sector is a recorded sector, defective sector, deleted sector or the like, the data part is a region for recording the desired information and the buffer part is provided as a buffer region for preventing the leading portion of the next sector from being recorded away even if a variation is caused in the rotation of the disc. The mark size of the data part is relatively small so as to record the information with a density which is as high as possible. Therefore, during recording it is necessary to effect the recording in synchronism with a certain frequency used as a reference and during playback it is necessary to effect the discrimination timing of playback signals in synchronism with the reference frequency; otherwise, there is the danger of failing to play back the desired recorded information due to any variations of the playback timing. The signals of this reference frequency are called as clock signals and the reference frequency is generally called as a clock frequency. A combination of these elements is referred to as a format.
Mainly, the below-mentioned formats are known.
(1) CLV system: Sectors of a given length are successively formed on spiral tracks from the inner side to the outer side and the rotational speed of a motor is controlled so as to obtain a rotational speed inversely proportional to the track radius thereby causing the linear velocity of the track to be recorded or played back (the relative velocity between the head and the disc) to become constant at any place on the disc. With this system, the clock frequency is constant and the recording laser beam intensity becomes the same at any position on the disc. Although the recording capacity is largest, there are disadvantages that the motor control is complicated and in this connection the access time is increased and so on.
(2) CAV system: This is a system in which the rotational speed of the motor (the disc) and the clock frequency are constant so that while the circuitry is simplified and the motor is also reduced in size, the recording capacity per track is dependent on the recordable minimum marksize along the innermost periphery of the disc recording area and therefore the overall recording capacity is smallest.
(3) Z-CAV system: This is one which incorporates the advantages of the above-mentioned two systems, that is, the large recording capacity of the CLV system and the easy motor control of the CAV system and it is so designed that with the motor (disc) rotational speed being constant, the disc recording area is divided into several zones and the clock frequency is fixed within each zone but closer the zones to the outer side higher is the clock frequency thereby increasing the recording capacity.
By virtue of this Z-CAV system, it is expected to make it possible to balance an increased recording density and an enhanced access performance at a high level.
However, the actual recording by the Z-CAV system is disadvantageous in that if, for example, an attempt is made to form marks of the same length at the same phases of the inner peripheral portion and the outer peripheral portion, the marks are caused to differ in position and length between the inner peripheral portion and the outer peripheral portion and this problem naturally causes deterioration of the discrimination quality of the signals recorded with a high density, thus causing failure to attain the essential aim of the Z-CAV system, i.e., the balancing of an increased recording density and an enhanced access performance at a high level. This phenomenon becomes more prominent relatively with increase in the recording density and it constitutes a serious obstacle for increase in the recording density. Also, the similar problem is caused, though in a less degree, in cases where the recording is effected by the CAV system.
With the Z-CAV system, the rotational speed of the disc is constant and thus the relative velocity of the laser beam and the medium or the linear velocity is varied depending on the recording position. In the Z-CAV system, generally the clock frequency is controlled so that it is in a substantially proportional relation with the linear velocity. On the other hand, while the temperature up and down characteristics for the heating and cooling processes of the medium due to the rise and fall of the laser beam also vary in dependence on the linear velocity, there is no proportional relation between the two. In other words, even if the clock frequency is controlled in proportion to the linear velocity, the temperature up and down characteristics do not vary in the same manner and therefore the manner of mark formation differs depending on the recording position.
While the fact that the temperature up and down characteristics for the heating and cooling processes of a medium due to the rise and fall of a laser beam vary in dependence on the relative velocity of the laser beam and the medium has been known as a phenomenon, the manner in which the two are related and the method of eliminating its effect have not been clarified.